Various systems are known in the art wherein materials are subjected to treatment within an insulated enclosure at temperatures substantially below those prevailing externally of the enclosure. Among such systems, for example, are included refrigerators and freezers for foods and other perishables as well as insulated chambers wherein articles are chilled to effect embrittlement of associated flash or coatings to facilitate removal of the embrittled portions by high velocity impact with particles of blasting media. To reduce heat leaks from such enclosure and heat transfer through the walls of the enclosure, the walls are thermally insulated and suitable gaskets and other sealing means provided at appropriate places. In installations wherein the temperature differential maintained within the enclosure and the external environment is relatively large, such as a .DELTA.T in the order of 200.degree.-300.degree. F. (=93.degree.-149.degree. C.) or more, there are construction problems associated with the expansion and contraction of structural elements leading, among other untoward effects, to warpage of the chamber walls and the access door thereto. These problems are particularly pressing in relatively large installations, because of the increased dimensional extent of thermal contraction and expansion at a given temperature ranges, and even more so in systems operated in the batch mode wherein the cold chamber is subjected to frequent opening and closing of the access door. This is the case, for example, with systems wherein workpieces, such as coated articles are subjected to cryogenic temperatures within an insulated chamber in a batch operation for embrittlement of the coating to facilitate removal of the coating. Such coating removal will entail a relatively short cycle time of generally less than ten minutes, involving frequent opening of the door between batches while the system is operating at a temperature in the order of about -200.degree. F. (-129.degree. C.). In commercial installations of large size, accordingly, the problems of structural stability despite frequent exposure to changing temperature, are correspondingly aggravated.
Also, in insulated chambers used for low temperature processes, the chamber insulation system needs to be sealed to preclude moisture condensation within the insulation, since such condensation will damage the insulation and render the same ineffective. Commonly, such low temperature insulation systems are protected from moisture condensation therein by lining the interior and exterior surfaces with a metallic material. Such inner metallic liner, of course, will exhibit thermal contraction and expansion as the system cycles from room temperature to cryogenic temperature. Accordingly, the amount of the thermal contraction usually limits this construction technique to relatively small sections, for example less than 60 inches (=152 cm.) in the longest dimension.
It should also be noted in the described prior insulation systems employing outer and inner metallic liners, the outer liner is usually reinforced to provide mechanical strength and stability to the insulated chamber assembly. The inner liner contracts and expands with changes in temperature to which it is alternately exposed, with some buckling and warping thereby resulting. The maximum length of the inner liner and the lowest operating temperature to which it is exposed, determine the total shrinkage of such liner. For example, in a freezer employing liquid nitrogen, wherein the inner liner of the cabinet wall is about 46 inches (=117 cm.) and is exposed to a temperature of about -280.degree. F. (-173.degree. C.), it has been found that there is a shrinkage of about 0.121 inches (0.31 cm.). The extent of buckling and warping of the inner liner under these conditions can be tolerated.
In a freezer employing CO.sub.2, on the other hand, where the lowest temperature of the operating range is about -90.degree. F. (=-68.degree. C.), an inner liner having a length of 98 inches (=249 cm.) will shrink a maximum of about 0.134 inches (=0.34 cm.), which extent of shrinkage would not effect buckling and warping of the inner liner beyond tolerable limits. However, when the combination of maximum length of the inner liner and the low operating temperature to which the liner is exposed, causes a total shrinkage in excess of the indicated limits, the inner liner will buckle and warp to an extreme degree, causing failure of the inner liner as well as the outer liner. Accordingly, in previous structures having metal panels subjected to extreme changes in temperature, construction was limited in size and temperature variation range to dimensions that will experience a total thermal change in length of no more than about 0.121 to 0.134 inches (=0.31 to 0.34 cm.).
As in the case of the stationary walls the insulated doors of cold chambers are also constructed with metallic outer and inner liners to protect the insulation therebetween from moisture condensation effects. When the metallic edge of the insulated door has a temperature variation from the "cold" face to the "warm" face, the edge of the door will bow. As the amount of the temperature difference increases the extent of distortion, the door will no longer effectively seal, and a gas leak then ensues. The magnitude of the distortion is also proportional to the size of the door; i.e. the length of the door edge. In large conventional insulated doors it is known to incorporate a massive frame external to the insulated door, to resist warpage of the door. Such external frames, besides being very heavy and costly, have only a limited value in controlling warpage of the door.
For efficient operation of systems of the type described, it will be appreciated that it is necessary to maintain a tight seal when the door to the chamber is closed, to prevent influx of warmer air and loss of cold cryogenic gas. To assure the required effective sealing the insulated doors must remain dimensionally stable and withstand the variation in temperatures to which the outer and inner faces of the door are exposed while the door is in its closed position, as well as the immediate change in temperature to which the inner face of the door is exposed between the frequent opening and closing of the door. Any warping or buckling beyond acceptable limits of the door itself and/or of the doorway of the chamber within which the door fits in closed position, will result in poor sealing of the chamber and entail additional expense in the cost of the cryogenic gas needed to maintain desired low temperature operation therein.
Among the several objects of the present invention is to provide a thermally insulated chamber with an insulated access door of novel construction, overcoming the problems heretofore encountered in or presented by prior art structures. A further object is to provide an improved construction adapted for use in large cryogenic chambers having insulated doors, which can be operated efficiently and effectively over a long period of useful life, without suffering excessive warping and deformation at low operating temperatures. A further object is to provide an improved thermally insulated chamber wherein effective hermetic sealing is maintained during operations therein and wherein the insulation within the walls and the door of the chamber is protected against moisture condensation therein.
The foregoing objectives are achieved by the novel construction and arrangement of the present invention.
While not limited thereto, the novel construction of the present invention has its most beneficial advantages in connection with systems employed in removal of flash and coatings by embrittlement and impact. In such systems the workpieces to be decoated, for example, are placed in a thermally insulated chamber wherein they are subjected to a low temperature gaseous atmosphere to effect the desired embrittlement and therein contacted with a high velocity stream of an impact medium such as plastic particles. The cryogenic chamber employed for such decoating operations requires a rigid frame to provide structural integrity for supporting the various mechanical systems in addition to the weight of the chamber itself, which systems include one or more throwing wheels for centrifugally hurling the impact media at high velocity against the workpiece, a plurality of conveyor systems for circulation of the impact media, and mechanically operated means for opening and closing the relatively heavy access door to the treating chamber. The structural framework of the chamber, accordingly, must be sufficiently rigid to maintain the position and alignment of the associated mechanical systems without excessive deflection or vibration.
Decoating operations in particular can be carried out in these cryogenic chambers relatively rapidly, such that in a batch operation the cycle time from one batch to the next needs to be no more than about six to eight minutes. Thus, the outer door to the chamber needs to be opened and closed frequently while the system is at operating temperature, in the order of about -200.degree. F. (=-129.degree. C.).
Examples of various prior art systems for cryogenic deflashing and decoating are described in U.S. Pat. Nos. 2,996,846; 3,110,983; 3,824,739; and Canadian Pat. No. 1,112,048; as well as in the copending U.S. patent applications hereinabove listed.